This invention relates generally to field effect transistors and more particularly to pseudomorphic field effect transistors.
As is known in the art, there are several types of field effect transistors (FETs) generally used at microwave and millimeter wave frequencies. These FETs include metal semiconductor field effect transistors (MESFETs) and high electron mobility transistors (HEMTs) each fabricated from Group III-V materials. What distinguishes a HEMT from a MESFET is that in a HEMT charge is transferred from an doped charge donor layer to an undoped channel layer. Generally the charge donor layer is a wide-band gap material such as AlGaAs, whereas the channel layer is a lower band gap material where band gap refers to the potential gap between the valence and conduction bands of the material There are generally two types of high electron mobility transistors One type is referred to simply as a HEMT whereas the other type is a pseudomorphic HEMT. The difference between the HEMT and the pseudomorphic HEMT is that in the pseudomorphic HEMT, one or more of the layers incorporated into the device has a lattice constant which differs significantly from the lattice constants of other materials of the device. Thus due to resulting lattice constant mismatch the crystal structure of the material forming the channel layer is strained. In a HEMT structure, charge is transferred from donor layer to an undoped channel layer. For Group III-V materials, the doped charge donor layer is comprised of a wide-band gap material such as gallium aluminum arsenide, whereas the channel layer is typically comprised of a lower bandgap material, such as gallium arsenide. A HEMT including an active region of AlGaAs and GaAs is unstrained, AlAs has a lattice constant .alpha.=5.6605.ANG., whereas gallium arsenide has a lattice constant .alpha.=5.6533.ANG.. Since these lattice constants are similar, the channel layer is unstrained.
In the pseudomorphic HEMT, the undoped gallium arsenide channel layer is replaced by a channel layer comprised of a lower bandgap material, such as gallium indium arsenide. Indium arsenide has a lattice constant .alpha.=6.0584. Since indium arsenide has a substantially different lattice constant compared to either gallium arsenide or aluminum arsenide, indium incorporation provides a crystal having a lattice constant which is substantially larger than the lattice constant of gallium arsenide or gallium aluminum arsenide. This lattice mismatch makes practical growth of such devices difficult and otherwise limits several advantages which would accrue to a device using GaInAs as the channel layer. For example, the use of gallium indium arsenide in a HEMT provides several performance advantages over gallium arsenide. Since gallium indium arsenide has a smaller bandgap than gallium arsenide, the conduction band discontinuity at the gallium aluminum arsenide/gallium indium arsenide heterojunction is increased thereby increasing the charged density transferred into the channel layer. Moreover, gallium indium arsenide also has a higher electron mobility and higher electron saturated velocity than gallium arsenide. Each of these benefits thus provides a pseudomorphic HEMT which can handle higher power levels, as well as, operate at higher frequencies with improved noise properties than a HEMT using gallium arsenide as the channel layer. Moreover, these benefits increase with increasing indium concentration (X) in the Ga.sub.1-x In.sub.x As layer.
Accordingly, a major objective in fabricating a high performance pseudomorphic HEMT structure is to maximize the amount of indium contained in the gallium indium arsenide layer. A problem arises, however, in increasing indium concentration. As mentioned above, gallium indium arsenide has a lattice constant which is larger than gallium arsenide or gallium aluminum arsenide, with the latter having substantially equal lattice constants. This disparity in lattice constants increases with increasing indium concentration. Thus, when gallium indium arsenide is disposed over the gallium arsenide, the film develops intrinsic stresses which induces a very high tensile strain in the gallium indium arsenide. For a gallium indium arsenide layer which is thicker than the so-called "critical thickness" of the GaInAs layer on gallium arsenide or gallium aluminum arsenide, this intrinsic strain causes the gallium indium arsenide film to be disrupted with formation of various types of crystal dislocations or defects. The presence of such crystal dislocations seriously degrade the electron transport properties of the GaInAs layer. For a gallium indium arsenide layer having a thicknesses less than the so-called critical thickness of the layer, the material is elastically strained without these dislocations forming. In the growth plane, the GaInAs layer takes on the lattice constant of the underlying gallium arsenide or gallium aluminum arsenide layer, whereas the crystal of the GaInAs is deformed such that in a plane perpendicular to the growth plane the crystal is expanded. This type of layer is termed "pseudomorphic" from which is developed the term pseudomorphic HEMT. With increasing indium concentration, the critical thicknesses at which the GaInAs layer forms crystal defects decreases. For example, for a channel layer comprised of gallium aluminum arsenide having the concentration Ga.sub.0.8 In.sub..2 As a layer thickness of approximately 100.ANG. is the maximum thickness. Layers thinner than approximately 100.ANG. are not attractive due to the increased importance of the quantum size effect which reduces the effective bandgap discontinuity. Thicknesses much above 100.ANG. result in the above-mentioned lattice dislocation problem.